Steels for Cryogenic and Low-Temperature Service

Sumário:

Carbon and alloy grades for low-temperature service are required to provide the high strength, ductility, and toughness in vehicles, vessels, and structures that must serve at –45°C and lower. Because a number of steels are engineered specifically for service at low temperature (about –100°C), selecting the optimum material calls for thorough understanding of the application and knowledge of the mechanical properties that each grade provides.

Carbon and alloy grades for low-temperature service are required to provide the
high strength, ductility, and toughness in vehicles, vessels, and structures
that must serve at -45°C and lower. Because a number of steels are
engineered specifically for service at low temperature (about -100°C),
selecting the optimum material calls for thorough understanding of the
application and knowledge of the mechanical properties that each grade
provides.

At temperature below ambient, a metals behavior is characterized somewhat by
crystalline structure. The yield and tensile strengths of metals that
crystallize in the body-centered cubic from iron, molybdenum, vanadium and
chromium depend greatly on temperature. These metals display a loss of
ductility in a narrow temperature region below room temperature.
The tensile strength of metals with face-centered cubic structures -
aluminum, copper, nickel and austenitic stainless steel - is more temperature
dependent than their yield strength, and the metals often increase in
ductility as temperature decrease.

Transformation occurring in compositions that are normally stable at room
temperature, but metastable at cryogenic temperatures, can greatly alter
their behavior. For example, the combination of gross plastic deformation
and cryogenic temperatures can cause a normally ductile and tough stainless
steel, such as 301, 302, 304, 321, to partially transform to bcc structure,
resulting in an impairment of ductility and toughness. A fully stable
stainless steel 310 cannot be transformed at cryogenic temperatures.

The 300 series steels offer a fine combination of toughness and weldability
for service to the lowest temperatures. In the annealed condition, their
strength properties are adequate for ground-based equipment but inadequate
for lightweight structures. For aerospace applications, fabricators can take
advantage of the alloys strain-hardening characteristics and use them in
highly cold-worked condition. The principal shortcomings of cold-worked
materials are: low weld-joint efficiencies caused by annealing during
welding and the transformation to martensite that occurs during cryogenic
exposure. Selection of fully stable grade type 310, overcomes the
transformation problem. Precipitation-hardening A286 stainless has even
higher strength when cold worked before aging.

The only alloy steel recommended for cryogenic service is 9% nickel steel.
It is satisfactory for service down to -195°C and is used for transport
and storage of cryogenics because of its low cost and ease of fabrication.
Other alloy steels are suitable for service in the low-temperature range.
The steels A201 and T-1 can suffice to -45°C, nickel steels with
2.25% Ni can suffice to -59°C, and nickel steels with 3.5% Ni to
-101°C.

Steel for Cryogenic Service: An Example

Designers of cryogenic assemblies base their stress calculations on the
room-temperature properties of the material. The reason is that it is the
highest temperature the material will encounter. And it stands that if a
higher-strength material that stands up to super cold conditions were
available, designers might specify it.

At 26°C austenitic stainless steel has tensile and yield strengths that
are 172 MPa greater than the corresponding strengths for type 304 stainless.
At -100°C its tensile and yield strength exceed those of type 304 by
550 MPa and 276 MPa respectively.

A grade with following chemical composition shows good mechanical properties
at cryogenic temperatures:

The material combination of high strength, good toughness, and weldability
should prompt designers to specify it for welded pressure vessels for the
storage of cryogens.

Steels for Low-Temperature

When designing low-temperature systems or equipment, the engineer finds that
notch toughness ranks high in importance, because a part or structure will
generally fail due to a notch or other stress concentration. Test results
measure the steels capacity to absorb energy, and thus signify its ability
to resist failure at points of local stress concentration.

Fatigue limit of steel also must be considered. At low temperatures, systems
are usually subjected to dynamic loads, and structural members to cycle
stresses. Examples include vessels that frequently undergo pressure changes
and large structures and mobile equipment that experience extreme stress
imposed by packed snow or high winds. Other considerations include heat
conductivity and thermal expansion.

Carbon steels have a better weldability, greater toughness, and higher
strenght with low coefficients of termal conductivity than alloy steels. The
A 516, one of the most frequently used group of carbon steels, have tensile
strengths ranging from 379 MPa to 586 MPa minimum. The big advantage of A 516
steels is their low initial cost.

Compared with A 516, A 442 class have higher carbon and manganese in plates
less than 25.4 mm thickness, and lower manganese beyond 25.4 mm. However,
applications for A 516 Grades 55 and 60 duplicate those of A 442. They are
easier to fabricate than A 442 grades because carbon content is lower.

Higher strength with good notch toughness is available in carbon steels
A 537 Grade A and A 537 grade B. Their can be earlier normalized or quenched
and tempered to raise yield and tensile strength and impact toughness beyond
those of the A 516`s. Table 1 shows mechanical properties at low
temperatures for some typical ASTM carbon steels.

Candidate material with high notch toughness for heavy-wall
pressure vessels

A543 Gr. 2

-107

690

793 - 931

14

Since a variety of low-temperature steels are available, the engineer must
consider the advantages each has to offer according to the application. The
cost-strength ratio is but one factor; others, such as welding and
fabrication costs, have equal or greater bearing on final costs. However,
heat-treated carbon grades are often used for low-temperature services.
Besides offering excellent low-temperature toughness plus fabricability,
these grades are lower in initial cost.

Pipeline Steels for Low Temperature Uses

Steels for natural gas pipelines must meet more demanding requirements than
that used for oil. For example they carry compressed gas at -25°C to
-4°C, making crack growth and brittleness a problem in the severe
artic environment. Achieving low-temperature notch toughness, grain size
control, and low sulfur content were among major problems in developing the
steel, particularly since economic feasibility had to be considered.

Hot-rolled steels present a good opportunity to cut both cost and weight if
the cost per unit strength could be reduced. As strength of high-strength,
low-alloy steels rise, toughness usually drops.

In steel alloyed with molybdenum, manganese and columbium, which is use for
these pipe-lines, molybdenum raises both strength and toughness. Carbon is
reduced to make columbium more soluble, and to improve weldability and impact
strength. Steels with small and large amount of columbium have similar
precipitation kinetics; higher strengths are produced by larger quantities
of columbium. Columbium also promotes hardenability, which is needed to
develop an acicular-ferrite microstructure. Manganese, along with molybdenum,
helps to inhibit transformation to polygonal ferrite on the steel.

Where sulphur cannot be kept low, however, rare earth additions will control
the shape of the sulfide inclusions. During hot working, grain refinement is
enhanced because columbium has a grain-boundary pinning effect. This effect
makes it possible to produce a highly substructured austenite prior
transformation, which helps in assuring transformation to fine grained
acicular ferrite.

Contributing to high strength and good impact resistance is the
transformation mechanism - austenite changes to fine-grained acicular
ferrite, which is further strengthened by the precipitation of columbium
carbonitride. Other advantages include good formability and most important,
excellent weldability.

Aside from pipeline, this steel can be used in the automotive, railroad,
heavy equipment, construction and shipbuilding industries, application areas
which the keynote is low cost per unit strength. Because of their inherently
good strength-toughness relationship, the manganese-molybdenum-columbium
steels may well satisfy this requirement.